1. Field of the Invention
A fiber-optic biosensor for assaying an analyte of interest.
2. Description of the Related Art
Concern over virulent pathogens contaminating the food supply has the potential to put large segments of the population at risk. The scientific estimates project that between 24 million and 81 million people become ill from foodborne diarrheal disease each year in the United States. The impact of this food contamination costs between $5 billion and $17 billion in medical care costs and lost productivity. Most toxicologists and food scientists agree these figures probably represent about 75% of the whole food safety risks, confirming that microbial pathogens are considered a serious hazard.
The impact due to Salmonella has been estimated at &gt;$4 billion from more than 3 million cases/year. The lack of success in effectively controlling the growth of these pathogens in foods suggests that the best approach would be early and rapid detection. Approximately 5 million analytical tests are performed annually in the U.S., which makes detection a high priority for diagnostic technology.
Traditional methods to identify and quantitate contaminants in foods include physicochemical, biological and serological tests. Most of these approaches lack sufficient sensitivity, selectivity, and take days to perform. There have been many attempts to provide faster and convenient detection methods. Most of these detection systems, although referred to as "rapid methods", still rely on culturing procedures to selectively amplify microbial populations.
The development of food immunoassays to improve this process has provided increased speed, simplicity, and effectiveness. However, these current procedures are expensive, usually require an enrichment/concentration step and still require hours to days for a final result. The adaptation of this immunoassay technology to biosensors has the potential to take immunoassays into the realm of rapid and reusable biosensors. The application of biosensors to the detection of Salmonella has the potential to contribute directly to the production and processing of safer and healthier food.
Biosensors are analytical devices which incorporate biologically active material in intimate contact with a transduction element to selectively and repeatedly detect analytes in products like food and food raw materials. The effective development of biosensors has become multidisciplinary, relying on biochemistry and biotechnology to provide the sensing elements through immobilization techniques and membrane technology, but also requires expertise in microelectronics, optical, acoustical and advanced signal processing. This dramatic combination can be effectively applied in the area of food safety, for the elimination of serious health risks by rapid detection of specific pathogens.
A compact fiber-optic evanescent-wave biosensor system which features an all-fiber optical design and red semiconductor-laser excitation has been developed. C. Zhou, P. Pivarnik, S. Auger, A Rand and S. Letcher, "A compact fiber optic immunosensor for Salmonella based on evanescent wave excitation", Sensors and Actuators B 42, pp. 169-175, 1997. Tapered fiber tips with immobilized antibodies for Salmonella attached were studied in different shapes and treatments and optimized. The system response for Salmonella concentration was established and could determine as low as 10.sup.4 colony forming units (CFU/mL) in 1 hour with a sandwich immunoassay format. Acoustic enhancement of the fiber-optic biosensor with ultrasonic manipulation of suspended particles has been shown to be an effective way to increase the sensitivity. C. Zhou, P Pivarnik, A. Rand, and S. Letcher, "Acoustic standing-wave enhancement of a fiber optic Salmonella biosensor", Biosensors and Bioelectronics 13, pp 495-500, 1998. Polystyrene microspheres (6-mm diameter) coated with immobilized antibodies were allowed to capture the antigens (Salmonella), which in turn captured antibodies labeled with fluorescent dye molecules. Then the entire structure was moved to the center of the acoustic cell, where the optical fiber with its cladding removed was located. The fluorescent signal was greatly increased over the signal without acoustic positioning. Multiple use of the system was also rapid because the fiber tip could be reused indefinitely (no antibodies were attached)--the flow cell just needed to be flushed out with a buffer solution and was ready for reuse.
Although the use of evanescent wave and tapered fibers provided average sensitivity and measurement potential, these systems did not achieve rapid and direct pathogen detection for food use. Currently the best available commercial system has a sensitivity of 10.sup.4 Colony Forming Units/ml (CFU/ml) and measurement potential of 10.sup.7 to 10.sup.8 CFU/ml . The time to complete an assay is usually 3 to 4 hours.